1. Field of the Invention
The invention is directed to a circuit, as well as a method for controlling the circuit, for generating an a.c. voltage from a plurality of unregulated voltage sources with temporally variable d.c. output voltages, such as photovoltaic installations, or parts of these installations.
2. Description of the Related Art
As used herein, a “photovoltaic installation” is intended to refer to an arrangement and electrical connection of a plurality of individual photovoltaic modules, as well as to partial installations thereof. Such photovoltaic modules are combined into photovoltaic installations, and as a condition of the type of their construction, have a compensating a.c. voltage which is dependent on the incoming solar radiation, which is therefore slowly temporally variable, i.e., it varys over time. These changes in the compensating a.c. voltage show time constants in the range of minutes or longer units of time.
To feed the output voltage of a photovoltaic installation into the public or the local electrical power grid it is necessary to convert the compensating a.c. voltage to an a.c. voltage of constant frequency and voltage by means of an a.c.-d.c. converter circuit.
An exemplary circuit for generating such an a.c. voltage is described in DE 10 2008 034 955 A1, which discloses a photovoltaic installation of a temporally variable output voltage, a level converter and an a.c.-d.c. converter. In the associated method, first and second capacitors of the level converter are charged independently of the natural voltage with respectively half the value of the rated intermediate circuit voltage.
A multitude of requirements exists for such circuits, which requirements are dependent on the particular application and which partially conflict with each other. For example, the individual photovoltaic modules should be charged linearly, if possible, i.e., within time intervals which are typical for output semiconductor circuits. Their output current should be as temporally constant as possible, i.e., within a time range of less than one second, or in a frequency range above 1 Hz. Also, the input voltage at the a.c.-d.c. converter circuit should be as constant as possible, and the transfer from the photovoltaic installation to the a.c.-d.c. converter circuit should take place at the highest possible voltage to keep output losses low. Finally, the entire circuit arrangement should operate at a high level of efficiency. At the same time, the photovoltaic installation, or the individual photovoltaic modules, should be operated so that they operate at the point of the highest output (MPP—maximum power point). A suitable regulation, so-called MPP tracking, is required to accomplish this.